Technical Field
The present disclosure relates to microelectronic fluid sensors on board integrated circuit chips.
Description of the Related Art
Fluids can be integrated with microelectronic circuitry in many applications including, for example, biomedical devices configured to perform physiological tests on samples of bodily fluids, and thermal actuators that eject fluid in response to changes in temperature.
Approximately 25-30 million people in the United States are diagnosed with type II diabetes requiring monitoring of blood glucose at least twice daily. A common example of a biosensor device that relies on a microelectronic fluid sensor is a disposable test strip used for measurement of blood glucose levels in diabetic patients as shown in FIG. 1. A typical blood glucose monitoring apparatus 80 includes a conventional disposable strip biosensor 82 and a portable electronic monitor 83. The biosensor 82 is made of a semi-rigid backing material 84 approximately an inch long, impregnated with an electrolytic chemical reagent 86 at one end and printed with electrodes 88 at the other end. The patient pricks a fingertip, applies a blood sample 89 to the electrolytic chemical reagent 86, and inserts the electrodes 88 into the portable electronic monitor 83. The electrolytic chemical reagent 86 conducts a current that is proportional to an amount of glucose in the blood sample 89. Current flow conducted via the electrodes 88 in the biosensor 82 closes a circuit when the biosensor 82 is inserted into the portable electronic monitor 83. The current in the circuit can then be measured by the portable electronic monitor 83. The portable electronic monitor 83 is configured with software that converts the current measurement into a numerical value that represents the blood glucose level. The portable electronic monitor 83 then provides a digital readout of the numerical value and stores the numerical value as blood glucose data in an electronic memory. By either recording or downloading the blood glucose data, the patient can track blood glucose values over time to adjust insulin dosage.
Use of an impregnated biosensor strip is problematic for several reasons. The chemical reagent 86 may degrade over time such that the biosensor strip has a finite shelf life and must be stamped with an expiration date. In addition, this type of biosensor strip is expensive, and available only on a prescription basis, as opposed to being an item that is sold over-the-counter. When the liquid sample is applied, sometimes the strip fails to take up enough of the liquid volume to make an accurate reading, and the test must be repeated, which incurs even more expense. Finally, the strips are disposable and cannot be re-used.
Another type of fluid detector that can be used to detect electrical properties of a fluid sample such as the blood sample 89, uses an open fluid reservoir instead of an impregnated reagent. An example of such a detector is a capacitive fluid detector as shown and described below with respect to FIGS. 2A, 3A, and 3B. Such a capacitive fluid detector transmits electrical signals through the fluid sample in the reservoir. The electrical signals can be compared against previous signals or an independent standard to detect changes. Changes in the electrical signals can indicate the presence or absence of fluid in a region between two electrodes, for example. Once presence of the fluid sample is detected, further changes in such signals can indicate fluctuating levels of fluid components that are charge-dependent such as glucose, electrolytes, or ions such as calcium, magnesium, potassium, and the like. The electrical signal data can then be sent to a microprocessor to calculate corresponding electrical properties of the fluid sample.
Depending on the design of the sensor, such a capacitive detection system may provide information regarding the presence of fluid, or the presence of certain components within the fluid, but not necessarily information regarding an amount of fluid present. For example, if the fluid participates in the circuit as part of a capacitive electrode rather than part of a capacitive dielectric, the capacitor geometry may not allow distinguishing between a small volume of fluid and a large volume. In the parallel plate capacitor sensor described above, the fluid is typically incorporated as a portion of one of the electrodes. However, where the fluid is incorporated as the dielectric, or a portion of the dielectric, it becomes possible to identify the fluid based on the dielectric constant. Such an arrangement is not feasible in the case of a capacitive sensor, however, because the dielectric, being sandwiched between two metal plates, is not easily accessible for introduction of a fluid sample by a user. Furthermore, capacitive sensor measurements may be affected by parasitic capacitances elsewhere in the circuit that are not actually related to the fluid sample and can therefore skew the test results. For at least these reasons, it may be desirable to have other types of fluid sensors available on an integrated circuit chip in addition to, or in place of, capacitive fluid sensors.